Centrifugal casting



June 13, 1%?

CENTRIFUGAL CASTING Filed June 2, 1964 INVENTOR. Daniel C. Shewmon ATTORNEY D. c. SHEWMQN 4,933

United States Patent 3,324,933 CENTRIFUGAL CASTING Daniel C. Shewmou, Wadsworth, Ohio, assignor to The llahcocii & Wilcox Company, New York, N.Y., a corporation of New Jersey Filed lane 2, 1964, Ser. No. 371,948 3 Claims. (til. 164--114) This invention relates to centrifugal casting and more particularly to a method of producing hollow metal articles by centrifugal casting. Such articles are usually cast by pouring molten metal into a mold and rotating the mold at high speed.

Previous attempts to improve the quality of centrifugal castings have not been satisfactory because, as recognized by the present invention, the inner surface of the casting solidifies while there is still molten metal containing gases and impurities beneath the hard inner surface. According to the present invention, the quality of centrifugal castings is improved by introducing into the mold a fluid or flowable material which is at a temperature above the solidification temperature of the metal, and which is in sufficient quantity to line the inner surface of the casting, to thereby maintain the inner surface of the casting in molten condition so that gases and impurities can work themselves out of the casting.

The various objects, features and advantages of the invention will appear more fully from the detailed description which follows, taken in connection with the accompanying drawing wherein:

FIG. 1 is an end view of a casting made according to a conventional centrifugal casting method; and

FIG. 2 is an end view of a casting made according to the centrifugal casting of the present invention.

The various attempts which have been made to improve the quality of hollow articles produced by centrifugal casting have not been entirely successful. Pipe and other hollow articles which are produced by presently known centrifugal casting methods invariably have a porous layer of metal adjacent the inner surface of the casting, wherein the impurities in the melt, and also voids caused by gases evolved during the casting process tend to concentrate. The impurities usually consist of slag occlusions, iron oxides, silicates, aluminates and other impurities resulting from reactions between dissolved gases and material additions to the molten metal, or they may come from interaction with the mold and the slag cover. This layer of unacceptable qualities must be removed at considerable expense and wastage of metal, sometimes 30% of the casting by volume, in order to obtain a satisfactory product.

By way of background, it should be understood that the impurities in the molten mass tend to undergo separation from the casting metal by action of the centrifugal forces associated with the process, and also by reason of a natural phenomen of the casting metal which tends to crystallize in the pure state while driving the impurities from the solidifying portion to the molten portion of the casting. More specifically, centrifugal force during rotation of the mold causes the heavier metal to displace the lighter impurities so that the impurities are collected on the inner surface of the casting, this inward movement being augmented by the self-purification of the casting metal as the casting progressively cools from its outer surface towards its inner surface.

It previously ha been proposed to introduce into the mold a fiuxing agent, such as glass, which coats the inner surface of the casting for the purpose of collecting impurities flowing inwardly, after which the contents of the mold are cooled and the frangible, hard glass-slag lining then removed. Despite the apparent merits of this improvement, it falls short of producing a sound metal cast- Patented June 13, 1967 ing because a relatively thick layer beneath the inner surface of the casting still has been found to contain unwanted voids and impurities. See FIG. 1 wherein a casting made according to a conventional casting method is illustrated. This unsound layer apparently results because the impurities and gases are unable to work themselves entirely out of the casting metal and into the molten flux.

The present invention recognizes that the difficulties with prior art methods can be attributed to the molten flux being introduced into the mold and there maintained at too low a temperature relative to the temperature of the molten metal which is being cast. In this connection, it should be understood that the variou fiuxing agents which would produce satisfactory results might cover a wide range of melting points as, for example from as low as 1000 F. to 3000 F. and above, depending on the melting point of the material being cast. For example, steel can be poured at temperatures ranging from 2750 F. to 3200 F; the melting range of nodular iron is from 2050 F. to 3150 F.; and the melting range of various bronze casting metals is from 1520" F. to 2100 F. Typical of the glass type fluxes which may be cited are soda lime glass which softens between 1800 F. and 2000 F., and glass high in silica which softens at about 3000 F.

It should be understood that if the flux lining is at a temperature below the solidification temperature of the casting metal, it will, in effect, chill the inner surface of the casting and thereby cause it to cool and solidify before the impurities including entrained gases have had an opportunity to escape from the interior of the casting. In other words, once the inner surface of the casting has solidified, the entrained impurities will be trapped within the metal being cast, and they will be unable to penetrate the hard inner surface which has already formed. It can be seen, therefore, that a fluxing agent cannot be entirely effective unless its temperature, before or after introduction to the mold, is substantially higher than the solidification temperature of the casting metal.

The difiiculty with presently known methods, as just described, is further complicated if the molten casting material contains a large amount of impurities. In this case, the impurities which, at the outset, work out of the casting into the molten flux produce a slag having very high melting point, and which quickly solidifies unless a disproportionately large amount of molten flux is employed so that the melting point of the resultant mixture will be only slightly affected by the addition of the impurities.

The present invention has been successfully practiced in making a centrifugally cast hollow pipe 7% inches in diameter with a wall thickness of 1 inch, as shown in FIG. 2, from medium carbon mild steel having the following analysis:

Percent Carbon -a .63 Manganese .47 Silicon .70 Phosphorous .004 Sulphur .013

the remainder iron plus incidental impurities and by employing as a fl'uxing agent a mixture in which 1.20 pounds of anhydrous borax was added to 6.80 pounds of common soda-lime window glass having the following approximate analysis:

The molten metal was poured into a rotating mold of circular cross-section at a temperature of 2960 F., after 3 which the glass was introduced at a pouring temperature of 3100 F. The resulting casting was strikingly free of impurities and voids when the flux lining was later removed after the casting had cooled and the inner surface of the pipe comparatively free of porous metal structure and pitting.

Glass and metal have dissimilar coefficients of expansion and therefore, upon cooling, the glass will separate from the inner surface of the casting. If it should be found that a particular lining does not adequately demonstrate this characteristic the separation can be accomplished while the solidified glass is still hot by spraying it with relatively cold water so that the resultant thermal shock will shatter the lining and leave a clean bright inner surface on the casting. Other means for removing the lining include striking the cooled casting so that the impact will loosen the lining.

The present invention can be practiced not only by introducing, as by pouring, a fluxing agent in the molten state, but also by introducing a high melting point ceramic or refractory material comprising finely divided discrete solids heated as aforesaid to a temperature above the solidification temperature of the metal. Thus, the fiuxing agent need only be in fluid or flowable state. Various refractory materials may be employed for this purpose but it should be borne in mind that the specific gravity of the fluxing agent, whether liquid or solid, should be less than that of the casting metal. The 'fluXing agent can be introduced before, after, or simultaneously with the introduction of the molten casting metal. Reference is made to a publication of the McGrawHill Book Co. entitled, Refractories, by F. H. Norton, 2nd Ed., printed 1942 by the Maple Press Co. of York, Pa., wherein will be found in Table 55 on pages 399 to 402 a listing of melting points and specific gravities of refractory materials from which a suitable fiuxing agent can be selected.

The invention further contemplates a method of centrifugal casting wherein the fluxing agent is maintained for a desired period at a temperature above the solidification temperature of the casting metal by means of supplementary heat, such as by introducing hot gases at appropriate temperatures.

It follows that the invention can be practiced by introducing a fluxing agent at a temperature below the temperature of the metal being poured and then effecting a temperature change which places the temperature of the fluxing agent above the solidification temperature of the metal. It is to be noted that during the casting process, irrespective of whether the fiuxing agent is molten or in particulate form, with or without supplementary heating of the flux, the reradiation of heat from within the confined flux mass itself will beneficially tend to maintain the flux temperature. Additionally, that portion of the casting metal adjacent the interior of the casting, and which is the last to solidify, will serve as a buffer with respect to the colder solidified outer portion of the casting, thus augmenting the 'reradiating effect of the fiux in maintaining the flux at a higher temperature than the molten or plastic inner portion during the casting process.

From the foregoing, it can be seen that an improved, more economical method for producing structurally sound, hollow articles by centrifugal casting has been provided by including the step of introducing into the interior of the rotating mold prior to solidification of the casting, a quantity of flux-like fiowable material sufficient to cover the inner surface of the casting, the temperature of the material according to the invention being above the solidification temperature of the casting metal so that the inner surface of the casting will be 4 maintained in a molten condition for a length of time sufficient to permit the entrained gases in the metal to escape from the inner surface of the casting, while the impurities in the casting metal collect in the flux lining prior to complete solidification of the casting. Thus, the quality of the casting is improved and wastage of metal is virtually eliminated; and it is especially advantageous in the casting of a hollow article having a thin wall.

Although the invention has been described in but several forms, it will be obvious to those skilled in the art that it is susceptible of application to various casting ma terials, and also of various changes and modifications, without departing from the spirit thereof as covered by the following claims.

What is claimed is: 1. The method of producing sound centrifugal castings which comprises providing a rotatable mold, rotating said mold, introducing molten metal to be casted into said mold during rotation thereof, providing a flowable refractory material in finely divided solid state when introduced into said mold, said material being preheated to a temperature above the solidification temperature of said casting metal and having a specific gravity less than that of said casting metal, introducing said material into said mold during rotation thereof and prior to solidification of the inner surface of said casting a sufficient quantity thereof to cover the inner surface of said casting and stopping the mold after at least said casting has solidified.

2. The method of centrifugally casting metallic tubular articles in a rotary mold which. comprises (a) introducing molten metal containing gaseous impurities into said mold While the mold is rotating to form by centrifugal action a tubular solidifying mass of molten metal tending to solidify initially at its outer and inner surfaces and to thereby entrap gaseous impurities in said mass, and

(b) maintaining an inner layer of said metal mass in molten condition for a length of time sufficient to permit substantially all of the gaseous impurities in the metal mass to escape from said inner surface and effecting unidirectional solidification of said casting from its outer surface to its inner surface by providing a layer of flowable refractory material at a temperature above the solidification temperature of the metal over the entire inner surface of the solidifying metal mass, said refractory material having a specific gravity less than that of the metal.

3. The method according to claim 2 including the additional step of maintaining said material at a temperature above the solidification temperature of said casting metal during the escape of gaseous impurities from said inner surface by supplying supplementary heat to the interior of said mold.

References Cited UNITED STATES PATENTS 1,831,310 11/1931 Lindemuth 22--200.5 1,908,170 5/1933 Naugle et al. 2-2200.5 2,124,445 7/ 1938 Ca'rrington 22-65 2,245,994 6/1941 McWane 22-65 2,710,997 6/1955 Krepps 22-200.5 2,737,696 3/1956 Shields 22215 FOREIGN PATENTS 1,269,898 7/1961 France.

I. SPENCER OVERHOLSER, Primary Examiner,

R. S. ANNEAR, Assistant Examiner, 

2. THE METHOD OF CENTRIFUGALLY CASTING METALLIC TUBULAR ARTICLES IN A ROTARY MOLD WHICH COMPRISES (A) INTRODUCING MOLTEN METAL CONTAINING GASEOUS IMPURITIES INTO SAID MOLD WHILE THE MOLD IS ROTATING TO FORM BY CENTRIFUGAL ACTION A TUBULAR SOLIDIFYING MASS OF MOLTEN METAL TENDING TO SOLIDIFY INITIALLY AT ITS OUTER AND INNER SURFACES AND TO THEREBY ENTRAP GASEOUS IMPURITIES IN SAID MASS, AND (B) MAINTAINING AN INNER LAYER OF SAID METAL MASS IN MOLTEN CONDITION FOR A LENGTH OF TIME SUFFICIENT TO PERMIT SUBSTANTIALLY ALL OF THE GASEOUS IMPURITIES IN 